1. Field
The present specification relates to RFID (Radio Frequency Identification) technology and, more particularly, to an RFID module that performs short distance wireless communication with a reader/writer, and to a portable device having this module incorporated thereinto.
2. Description of the Related Art
RFID technology is basically a technology that recognizes a moving body in a non-contact manner. An RFID system that performs short distance wireless communication among readers/writers and RFID modules is usually formed by using electromagnetic coupling, electromagnetic induction, radio waves, or the like.
As RFID modules of an electromagnetic induction method, card-shaped RFID modules that are used for various applications, such as electronic money, commuter passes, or employee ID cards, have become popular. In recent years, regarding functions of non-contact IC cards, portable devices, such as mobile phone terminals having such function incorporated therein, have been put onto the market.
In RFID systems, in such a function incorporated in portable devices in which metal is heavily used, the problem of “phase inversion null” has been known.
The term “null” in an RFID system refers to a phenomenon in which communication cannot be performed in spite of the fact that there is a sufficient distance (within a communicable range) to receive electric power necessary for communication, and there are various factors why such a null might occur. Phase inversion is one such factor. Null caused by phase inversion will be referred to as phase inversion null.
The term “phase” used herein refers to the phase of a waveform after an envelope is extracted from a carrier wave that has been modulated by amplitude shift keying (ASK). Since this phase differs from the phase of a 13.56 MHz carrier wave, hereinafter, this phase will be referred to as an “envelope phase” to avoid confusion.
Short distance wireless communication of an electromagnetic induction method is performed as a result of a device called a reader/writer (hereinafter will also be referred to as an R/W) that sends a carrier wave being magnetically coupled with an RF module that does not output a carrier wave by itself. In communication from an RF module to an R/W, the inversion of an envelope phase can easily occur depending on the state of magnetic coupling. Since an RF module typically has a card-like shape, hereinafter, the RF module will also be simply referred to as a card. However, in the manner described above, in a case where an RF module is incorporated into a portable device, of course, the RF module will not have a card-like shape.
When the envelope phase is inverted, the size of the amplitude in ASK modulation, and the relationship of high/low as digital data are inverted. Since a specific RFID protocol is created in such a manner that communication can be normally performed even if inversion occurs, the fact that the envelope phase is inverted in itself does not pose a problem. However, in a process at which the envelope phase becomes inverted, a point in which the difference between the large and small carrier wave amplitudes becomes zero with respect to high/low of the digital data exists. At this time, since the R/W cannot demodulate response data from the card, a null occurs.
FIG. 1 illustrates the relationship between the inversion of an envelope phase and the demodulation possible/not possible state. The waveform on the upper side of FIG. 1 represents data that is being transmitted by a card, the data being reflected in changes of the antenna current of the card. The waveform on the lower side of FIG. 1 represents a carrier wave waveform that appears in an antenna of an R/W. It can be seen that as the envelope changes from left to right in FIG. 1, inversion of the envelope phase occurs. At this inversion, the state of “demodulation is not possible” occurs.
Here, a description will be given briefly of a mechanism in which an envelope phase is inverted.
Data transfer in the direction from a card to an R/W is performed by a “load modulation method” that causes the load resistance of a card-side antenna to change. In such a load modulation method, high/low representation as digital data is performed by turning on/off an FET for load modulation, which is incorporated into an RFID circuit block (normally, chip configuration) on the card side. (Because Manchester code is used, “1” is indicated by high→low, and “0” is indicated by low→high). Hereinafter, the FET for load modulation will also be referred to as a load switch (load SW).
Since the R/W and the card in the middle of communication are magnetically coupled with each other, the change in the antenna current of the card due to the load SW is detected as an amplitude change of the carrier wave waveform in the antenna of the R/W. For this reason, the R/W performs demodulation by envelope detection in the same manner as for demodulation of an ASK modulation wave.
A description will be given of a model such that a state in which changes in the antenna current of a card are converted into changes in the antenna voltage of the reader/writer is simplified by magnetic coupling. FIG. 2(a) illustrates main circuit units of an R/W and a card. In this figure, an RFID module of a non-contact IC card is referred to as a “card” for the sake of convenience. Inside the card, only transmission-related portions are shown, and the illustration of the other elements is omitted.
Blocks of “TNS” and “RCV” in the R/W indicate a transmission unit and a reception unit, respectively. FIG. 2(b) illustrates an equivalent circuit in which an R/W and a card that are magnetically coupled are simplified. The voltage V1 of the R/W shown in FIG. 2(b) corresponds to a voltage that is generated in the antenna of the R/W.
FIG. 2(b) is a circuit diagram illustrating an equivalent circuit in which an R/W and a card that are magnetically coupled are simplified. In this figure, V1 corresponds to a voltage that is generated in the antenna of the R/W.
When V1 is represented by a circuit equation, V1 can be described as in following equation (1).
                                                                        V                .                            1                        =                                          (                                                      jω                    ⁢                                                                                  ⁢                                          L                      1                                                        +                                      R                    1                                                  )                            ·                                                I                  .                                1                                                          ︸                                          Voltage                ⁢                                                                  ⁢                generated                ⁢                                                                  ⁢                at                            ⁢                                                          ⁢                                                antenna                  ⁢                                                                          ⁢                  end                  ⁢                                                                          ⁢                  by                  ⁢                                                                                                    ⁢                                                                                  ⁢                                                                                                  ⁢                                                            I                      .                                        1                                                  =                a                                                    +                              (                                          -                jω                            ⁢                                                          ⁢                              M                ·                                                      I                    .                                    2                                                      )                                ︸                                          Voltage                ⁢                                                                  ⁢                generated                            ⁢                                                          ⁢                              at                ⁢                                                                  ⁢                antenna                ⁢                                                                  ⁢                end                ⁢                                                                  ⁢                by                            ⁢                                                          ⁢                                                          ⁢                                                                    I                    .                                    2                                =                b                                                                        [                  Math          .                                          ⁢          1                ]            
It can be said from this equation (1) that the voltage V1 is an additive combination of a voltage a generated by current I1 and a voltage b generated by current I2 that flows through the card-side antenna. As a result of this, since I2 changes by the ON/OFF of the load SW, the conveyance of information by envelope detection is made possible.
Furthermore, the relationship between I1 and I2 in this equivalent circuit is represented by the following equation (2).
                                          I            .                    1                =                                            I              2                                      ω              ⁢                                                          ⁢              M                                ⁢                      (                                          ω                ⁢                                                                  ⁢                                  L                  2                                            -                              1                                  ω                  ⁢                                                                          ⁢                                      C                    2                                                              -                              j                ⁢                                                                  ⁢                                  R                  2                                                      )                                              [                  Math          .                                          ⁢          2                ]            
As a result, it may be said that the phase difference between I1 and I2 is influenced by the relationship among L2, R2, and C2 (≈resonance frequency of the card).
Here, the following point become a problem. That is, regarding the phase difference between the voltage waveforms a and b, the voltage a is influenced by the relationship between L1 and R1, and the voltage b is influenced by the relationship among L2, R2, and C2. For this reason, as a result, the voltage V1 becomes such that two sine waves having a mutual phase difference are additively combined. When two sine waves are to be additively combined, if the mutual phase relationship is in phase, the levels of the waveforms are directly added. If the phase relationship is in opposite phase, the levels of the waveforms are subtracted. In an intermediate state between in-phase and opposite phase, a phase relationship in which the level of the waveform does not change exists, with the result that the change of the amplitude of ASK after combination is lost.
FIG. 3 illustrates an example of a waveform using a channel (ch) additive combination function of an oscilloscope as a reference for the purpose of explaining a voltage V1 that is generated in an antenna of an R/W. This illustrates that the waveform of ch1 in which the voltage a is assumed and the waveform of ch2 in which the voltage b is assumed are combined. The range of the time axis is changed to show the same waveform in the upper half and the lower half of FIG. 3 show the same waveform. The upper half shows a waveform in the carrier wave range (50 nsec/div), and the lower half shows a waveform in the ASK modulation range (2 μsec/div). Furthermore, the left portion of FIG. 3 represents the state at the time when the waveforms are completely in opposite phase, the right side portion of FIG. 3 represents the state at the time when the waveforms are completely in phase, and the central portion represents the halfway state (null) that is intermediate between them.
Next, a description will be given of a relationship between a resonance frequency and a position at which a null of a portable device having a non-contact IC card function easily occurs (that is, incorporating an RFID module therein).
What particularly becomes a problem in the case of a portable device in which a metal is heavily used is a variation in the self-inductance (L1) of the R/W antenna. A magnetic flux forming a self-inductance (L1) of an R/W antenna is cancelled by an eddy current that is generated in a metal surface (accurately, a conductor of a plane shape in general), such as the housing of an electronic apparatus or the GND plane of a substrate. For this reason, the self-inductance of the R/W antenna as the portable device comes close to the R/W antenna is greatly decreased. At the same time, the phase relationship of the voltage b with respect to the voltage a varies to the leading side.
In a case where the amount of variation of the carrier wave phase cannot be contained within a range in which a null does not occur, communication cannot be performed in the vicinity of a position at which the portable device is in close contact with the R/W.
FIG. 4 is a graph indicating actually measured examples of variations of the L value when a portable device is made to come close to a loop antenna. The horizontal axis of this graph represents the distance (mm) from the portable device up to the loop antenna, and the vertical axis represents the inductance value (μH) of the self-inductance (L1) of an R/W antenna. Since the arrangement and the area of the metal body differs depending on what portable device is made to come close to the R/W, the magnitude of the influence that is exerted on the opposing R/W device differs. However, it can be seen that even if the devices are different, the inductance value tends to decrease with a decrease in the distance.
In the R/W, when, in particular, the resonance frequency (hereinafter denoted as f0) of the portable device is high as a result of the inductance of the R/W antenna being decreased when the portable device comes close, since the phase relationship of the voltage b with respect to the voltage a is such that both voltages are in the leading direction, unfavorable conditions coincide one another, and a null is easily generated.
With such a mechanism, the relationship between f0 of the portable device and the communication distance at which a null easily occurs has a tendency shown in the graph of FIG. 5.
FIG. 5 is a graph illustrating the relationship between the resonance frequency of a portable device and a communicable area. The horizontal axis of this graph represents the resonance frequency f0 (MHz) of the portable device, and the vertical axis represents the communication distance (mm), that is, the distance from the portable device up to an R/W. The right bar of a pair of adjacent bars for each value of the resonance frequency of the portable device indicates the communicable position of the portable device with respect to the R/W in which f0 is low. The left bar indicates the communicable position of the portable device with respect to the R/W in which f0 is high. The black bar portion of the lower right area of the figure indicates the position at which a null has occurred.
With regard to the resonance frequency f0 of the R/W, variations due to individual differences among individual devices are assumed. As can be seen from FIG. 5, communication is not possible between an area R1 of a more distant place for the combination of an R/W whose resonance frequency f0 is low and a portable device whose resonance frequency f0 is low and an area R2 of a more distant place for the combination of an R/W whose resonance frequency f0 is high and a portable device whose resonance frequency f0 is high. In addition to this, for the combination of the R/W whose resonance frequency f0 is high and the portable device whose resonance frequency f0 is high, it is easier for a null to occur in an area R0 in the vicinity of close contact with the R/W. In particular, since the occurrence of a null in the area R0 in the vicinity of this close contact is a phenomenon contrary to the intuition of the user who would expect “the smaller the distance, the easier communication will be”, this is directly connected to the worsening of usability.
Therefore, normally, it is necessary to strictly manage the resonance frequency f0 of the portable device so that the communication distance thereof satisfies the specification and falls within the range (hereinafter referred to as an allowable range) in which a null does not occur. However, the resonance frequency f0 is also influenced by not only variations in the parts such as a loop antenna and a tuning capacitor, but also by mechanical structure-like variations and assembly variations in manufacturing steps, such as the positional relationship between a loop antenna and the GND plane of a substrate. For this reason, the variation range of the resonance frequency f0 due to the mass production of portable devices often exceeds the allowable range thereof. In order to manage the variation range to be within a narrow band, it is necessary to sacrifice the cost of parts and takt time in manufacturing steps, such as the conversion into an f0 adjustment circuit based on more number of bits or adjustments using a trimmer capacitor.
Hitherto, regarding the null problem, it is comparatively easy to take measures on the reader/writer device side than to perform null measures on the card side, and measures within the reader/writer device have already been put into the market. On the other hand, reader/writer devices for which measures have not been taken have already become popular in large quantities, and it is preferable that a null is not generated in communication with those reader/writer devices. For this reason, there has been a demand for null measures on the card side.
As a method of the related art for taking measures on the card side, there is a technology disclosed in PTL 1. FIG. 19 illustrates an overall configuration of a portable device according to such a method of the related art. In this method of the related art, a level detection unit 15 provided in a mobile terminal detects the level of the antenna excitation voltage of a loop antenna 11, and ON/OFF control of an FET 16 is performed in accordance with the level detection unit output, thereby selectively adding the capacitance of a capacitor 17 to the capacitance of the capacitor 12 forming a resonance circuit together with the antenna 11. With this arrangement, control of decreasing the resonance frequency when the mobile terminal comes close to the R/W is performed.
As the internal configuration of the level detection unit 15, in general, processes are performed as follows with the circuit configuration shown in the figure. That is, the excitation voltage of the loop antenna 11 is converted into a DC voltage by a rectification diode 152 in a rectification unit 151, and an increase in this DC level is detected by using a comparator 153 or the like. An FET switch 16 is subjected to ON/OFF control in accordance with the detection output of the comparator 153.